Field of the Invention
The invention relates to an automatic gain control circuit and, more particularly, to such a circuit which is adapted for use in high frequency amplifier circuits such as those used in tuners or the like.
Description of the Prior Art
Radio-frequency amplifier circuits such as those used in tuners have used bipolar type automatic gain controlling transistors (hereinafter referred to merely as AGC transistors) or metal-oxide-semiconductor type field-effect transistors (MOS-FET) which exhibit a relatively superior cross modulation characteristic. However, such transistor elements have been insufficient and have had numerous disadvantages such as described below with reference to FIG. 1, which figure shows the change in interference level causing 1% cross modulation as a function of the gain controlled variable. An AGC transistor used in common emitter fashion exhibits an inferior cross modulation characteristic at zero gain control variable as shown by the curve 1 and an AGC transistor used in common base fashion exhibits a cross modulation characteristic not improved at an increased gain controlled variable as shown by the curve 2. Also, a MOS-FET exhibits a cross modulation characteristic not improved much at an increased gain controlled variable as shown by the curve 3. The insufficient cross modulation characteristic causes cross modulation and a manual switched attenuator typically is provided in such a device as a television set so as to attenuate the input signals.
With the increase in technical level, inexpensive pin junction diodes having a sufficiently superior high-frequency characteristic have become available and a combination of an attenuator employing such pin junction diodes and a high frequency amplifier employing a MOS-FET permits the improvement of the cross modulation characteristic as shown by the curve 4.
On the other hand, such pin junction diodes have another disadvantage such as described below with reference to FIG. 2, which figure shows the change in noise factor (NF) as a function of gain controlled variable. The use of such pin junction diodes results in noise factor deterioration in proportion to the increase in gain controlled variable as shown by the curve 5, causing snow noises to appear in the picture of a television set in an area of a low radio-field intensity although such noise factor deterioration does not create any trouble in an area of high or intermediate radio-field intensity because of the high input signal level.
With the use of the AGC transistor or MOS-FET, a low noise factor of 2 to 5 dB in the range 5 to 10 dB of gain controlled variable appears as shown by the curve 6 of FIG. 2 and the noise factor deterioration causing snow noises found in the attenuator employing the pin junction diodes is not experienced.